7 Compressors

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Compressors & Gas Compression

Categories and Types

Compression Process

Compressor Characteristics Key Design Parameters

Calculation Methods

Specification Data Sheet Selection Guidelines

Control Systems

Typical operating Problems


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Positive Displacement

Reciprocating (Piston, Diaphragm) Rotary Type (Screw, Lobe, Slidiong Vane

Dynamic Centrifugal (Radial and Axial)

Blowers

Categories and Types


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Compressors & Gas CompressionCategories and Types


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Compressors & Gas CompressionCentrifugal Compressor


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Compressors & Gas CompressionAxial Compressor


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Compressors & Gas CompressionRanges of Application


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Compressors & Gas CompressionCompression Process

Gas compression is a thermodynamic process where changetakes place in the physical state of the gas

Compression adds energy to the gas resulting in pressure-volume changes defined by ideal gas laws

Compression take place under conditions defined: Adiabatic: no heat added or removed from systems

Isothermal: constant temperature in system Polytropic: heat added or removed from system

Compression of real gases in actual compressors deviatefrom conformance with ideality, usually significantly,affecting compressor design.


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Compressor Characteristics

Capacity/Head

Performance

Terminology


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Reciprocating Compressor

Performance Diagram

Terminology Piston Displacement Clearance Volume

Volumetric Efficiency Pressure Ratio Rod Loading


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Centrifugal Compressor

Performance Curves

Terminology Operating Point Surge Point

Stonewall Stability Turndown


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Compressors & Gas CompressionCentrifugal Compressor Performance


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Key Design Parameters

Capacity

Gas Properties Pressure Head Power Efficiency Multi-Stages


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Flow Rates Normal Maximum Minimum

Design Capacity

Capacity


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Composition Contaminants Molecular Weight MW Specific Heat Ratio Cp/Cv Compressibility

Gas Properties

& G


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10C

38C

66C

93C121C

& G


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C & G C i


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C & G C i


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100F = 560R: 560/549 = 1.02100F = 311K, 549R = 305K: 311/305 = 1.02

PV = ZmRT/MWP=100psia = 6.89 bar a T=100F = 37.8C = 310.9K = m/V = P(MW)/(ZRT)= 6.89E5x34.27/(0.946x8314x310.9)

= 9.7kg/m3= 0.61lb/ft3

C & G C i


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0.9730.077 1.02

C & G C i


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0.88

C & G C i


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Available vs. Required Head Available Head is Compressor Related

H(Available) = CV2/g C = Pressure Coefficient (0.55)

Required head is System-Related

Head

H(Required)

C & G C i


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For centrifugal compressors the followingmethod is normally used:

First, the required head is calculated.Either the polytropic or adiabatic efficiencyis used with the companion head.

Horsepower Calculation

C & G C i


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Horsepower Calculation

Where:Z = Average compressibility factor: using 1 will yield

conservative results

R = 1544/(mol weight)T1 = Suction Temperature, RP1, P2 = Suction, discharge pressures, psiaK = Adiabatic exponent, (N-1)/N = (K-1)/(KEp)Ep = Polytropic EfficiencyEA = Adiabatic Efficiency

C & G C i


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Horsepower CalculationThe polytropic and adiabatic efficiencies are related as follows:

From Polytropic Head:

HP = WHpoly/(Ep 33000)

From Adiabatic Head:

HP = WHAD/(EA 33000)

Where:

HP = Gas Horse PowerBHP = Brake HorsepowerW = Flow, Lb/min

BHP = HP/Em

C & G C i


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Efficiency

Hydraulic Efficiency

Adiabatic Polytropic

Volumetric Efficiency Reciprocating

Mechanical Efficiency Drivers

C & G C i


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Approximate polytropic efficiencies for centrifugal and axial compressors

C & G C i


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Compressors & Gas CompressionTemperature Rise

Temperature ratio across a compression stage is:

T2/T1 = (P2/P1)(K-1)/K Adiabatic

T2/T1 = (P2/P1)(N-1)/N Polytropic

Where:

K = Adiabatic exponent, Cp/CvN= Polytropic exponent, (N-1)/N = (K-1)/KEpP1, P2 = Suction, discharge pressures, psiaT1, T2 = Suction, discharge temperatures, REp = Polytropic efficiency, fraction

C m & G C m i


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The usual centrifugal compressor is uncooled internally andfollows a polytropic path.

Temperature must often be limited to: Protect against polymerization as in olefin or butadiene

plants At T > 230-260C, the approximate mechanical limit,

problems of sealing and casing growth start to occur.

High temperature requires a special and more costly machine.Most multistage applications are designed to stay below 250-300C

C mp & G C mp i n


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Intercooling can be used to hold desired temperatures for highoverall compression ratio applications.This can be done between stages in a single compressor frame orbetween series frames.

Sometimes economics rather than a temperature limit dictateintercooling.

Sometimes for high compression ratios, the job cannot be donein one frame. Usually a frame will not contain more than 8 stages

(wheels). For many applications the compression ratio across aframe is about 2.5 4.0

The maximum head that one stage can handle depends on gasproperties and inlet temperature. Usually this is about 2000 to

3400m for a single stage.

C mpress rs & Gas C mpressi n


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Compressors & Gas CompressionSurge Controls

A centrifugal compressor surges at certain conditions of low

flow.

Surge control help the machine to avoid surge by increasing flow.

For an air compressor, a simple spill to atmosphere is

sufficient.

For a hydrocarbon compressor, recirculation from discharge

to suction is used.



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Compressors & Gas CompressionSurge Controls

There are many types of surge controls.

Avoid the low-budget systems with a narrow effective range,

especially for large compressors.

Good systems include the flow/P type.

The correct flow to use is the compressor suction. However, a

flow element in the suction can rob excessive horsepower.

Therefore, sometimes the discharge flow is measured and the

suction flow calculated within the controller by using pressure

measurements. The compressor intake nozzle is also sometimes

calibrated and used as a flow element.



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Compressors & Gas CompressionCompressor Calculation Method

Define gas properties: MW, Cp/Cv, Z 1

Define inlet conditions: Temp & Press.

Calculate gas flow rate: Normal and Design 1 Establish total discharge pressure.

Calculate compression ratio and number of stages

Define selection & polytropic efficiency

1. At inlet conditions



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Compressors & Gas CompressionCompressor Calculation Method contd

Calculate heat capacity factor M

Calculate required polytropic head

Calculate hydraulic gas horsepower

Calculate discharge temperature

Calculate total brake horsepower

Estimate inter-stage cooling requirement



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Compressors & Gas CompressionCompressor Calculation Example 1:

Calculate compressor required to handle a process gas at thefollowing operating conditions: Inlet press and temp at 20 psiaand 40F. Discharge pressure of 100 psia. Gas rate 2378

lb.mol/hr of the following composition and calculatedproperties:

Mol% Mol/h Mol.Wt

Cp Tc Pc

Ethane 2 48 30.1 0.60 11.96 0.24 550 11 708 14Propane 95 2259 44.1 41.9 16.55 15.70 666 633 617 587

Butane 3 71 58.1 1.74 22.50 0.68 766 23 551 17

Total 100 2378 44.24 16.62 667 618



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Compressors & Gas CompressionCompressor Calculation Example 1: contd

Inlet flow:

Weight flow = 2378 x 44.24/60 = 1753 lb/min

Pr = 20/618 = 0.0324, Tr = (40+460)/667 = 0.75Compressibility factor Z = 0.97 (from generalized Z chart)

Density = (MW x P1)/(10.73 x T1 x Z)

= (44.24 x 20)/(10.73 x (40 + 460) x 0.97)= 0.17 lb/cu.ft

Inlet volume = 1753/0.17 = 10 310 cu.ft/min

Calculation:



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Heat Capacity Factor

k = Cp/Cv = Cp/(Cp 1.99) = 16.62/(16.62 1.99) = 1.137M = (k-1)/(kEp)

Assume Ep = 77%:M = (1.137 1)/(1.137 x 0.77) = 0.156

Calculation:



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Polytropic Head, Hp

Calculation:

= 0.97 x (1545/44.24) x (40 + 460)/0.156 x [(100/20)0.156 -1]= 30 988 ft



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Discharge Temperature, T2

T2 = T1(P2/P1)M= 500(5)0.156= 643R= 183F

Gas Horsepower (GHP) & Brake Horespower (BHP)

GHP = W . Hpoly/(33000Ep)= 1753 x 30988/(33000 x 0.77)= 2140

BHP = 2140/0.98 = 2180 (Assume Mechanical Eff. = 98%)

Calculation:



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Compressors & Gas CompressionExample Calculate the Brake Horsepower for the following Compressor:

07TI001 07TI002 07TI004 07TI005 07TI006 07T1008 07TI010 07TI012

22 99 50 124 55 139 57 65C C C C C C C C

07PIC004 07PI005 07PI006 07PI007 07PI017 07PI009 07PI013 07PI011

2418 4300 4250 7700 7643 14 14 14.6

kPa g kPa g kPa g kPa g kPa g MPa g MPa g MPa g

07FI003 07FI004

107 353

kmn3/h kmn

3/h

11497

11464

0.0%

0.0%

20.0 0.1% 08AI004

87.0 2.5%

kmn3/h KNM3/H 15.0% % Argon

Argon

H2 65.6 Purge

0.0% to Flare

N2 21.4

Actual Speed =

Reference Speed =

RECYCLE

STAGE 1 STAGE 2 STAGE 3 STAGE 4

C3030 C3031HC02

HC06

HC02

HC08

C3032

C3034

HC41

TO NH3 REACTOR

Red Blocks = Local Readings (necessary for MW calculation)



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Compressors & Gas CompressionExample Calculate the Brake Horsepower for the following Compressor:

Calculate Gas Mixture Properties

Composition: H2 = 65.6/(65.6+21.4) = 75.4 vol%

N2 = 100 75.4 = 24.6 vol%

Composition Mole% Mole Wt MW mass% Cp MWHydrogen 75.4 2 1.51 18.0 14.3 2.57Nitrogen 24.6 28 6.89 82 1.04 0.85

Total Gas Mix 100.0 8.40 11.04 3.42

Use Z = 1 for conservative results



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Compressors & Gas CompressionExample Calculate the Brake Horsepower for Compressor: Contd

Lets look at the first stage:

First calculate Polytropic Head:

T2/T1 = (P2/P1)(N-1)/N

ln(T2/T1) = (N-1)/N ln(P2/P1)

(N-1)/N = ln(T2/T1)/ln(P2/P1)

= ln(372/295)/ln(4400/2518)

= 0.416

Hpoly = 1 x (8.314/8.4) x 295 x ((4400/2518)0.416 -1)

0.416

= 183.4 kJ/kg

T1 = 22C = 295KT

2

= 99C = 372KP1 = 2418 kPag = 2518 kPa aP2 = 4300 kPag = 4400 kPa a



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First stage:

(N-1)/N = (K-1)/(KEp)

Ep = (1.4 -1)/(1.4 x 0.416)

= 0.69

W = (107 000/22.414) x 8.4 = 40100kg/h = 11.14 kg/s

Cp/Cv = Cp/(Cp-R)= 3.42/(3.42-8.314/8.4)

= 1.4



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First stage:

Gas Horsepower = W . Hpoly/Ep

= (11.14 x 183.4)/0.69

= 2960 kJ/s= 3.0 MW

Similar for stage 2, 3 and Recycle:

GHP(stage 2) = 2.9MW

GHP(stage 3) = 3.3 MW

GHP(recycle stage) = 1.0 MW

Total GHP = 3.0 + 2.9 + 3.3 + 1.0 = 10.2 MW

A good assumption for Mechanical Efficiency = 95%

BHP = 10.2/0.95 = 10.6 MW



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Date post:	03-Apr-2018
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7 Compressors

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